The invention relates to a device for regulating the rate at which fuel is taken from a fuel circuit of an aircraft propelled by an engine, and particularly but not exclusively by a gas turbine engine.
It is common practice for fuel to be fed to a gas turbine airplane engine via a positive displacement pump (e.g. a gear pump). The pump receives fuel from a fuel circuit of the airplane and delivers a flow of fuel in particular for feeding the combustion chamber of the engine.
A positive displacement gear pump presents the particular feature of delivering fuel at a rate that is proportional to its speed of rotation. Since the pump of an airplane engine is generally driven by the accessory gearbox (AGB) that is coupled to a shaft of the engine (e.g. the high pressure shaft in a two-spool turbomachine), the rate at which it delivers fuel to the combustion chamber is mechanically proportional to the speed of rotation of the engine.
Unfortunately, the requirement of the combustion chamber of the engine for fuel injection is not linear relative to the speed of rotation of the engine. In addition, given its particular manner of operation, a positive displacement gear pump needs to be dimensioned so as to be capable of delivering a quantity of fuel that is sufficient for ensuring that the combustion chamber of the engine operates during any stage of flight, and in particular during takeoff or when relighting the engine. As a result, during most stages of flight of an airplane, the quantity of fuel that is taken by the pump is greater than the quantity needed to ensure operation of the combustion chamber, so the fuel that is not used needs to be returned to the fuel circuit by a recirculation loop.
In addition to the complexity of providing circuits for returning the unused fuel to the fuel circuit, such as solution for feeding the engine with fuel presents the drawback of drawing mechanical energy from the engine shaft driving the pump that is greater than real requirements, with this surplus mechanical energy not being available for driving other equipment in the engine or the airplane. In addition, the presence of a recirculation loop has the drawback of heating the fuel and thus contributing in negative manner to the thermal behavior of the system.
The use of a variable-geometry pump (i.e. a pump having a rate that can be regulated under hydraulic control) enables the above problems to be solved in part. Nevertheless, such a pump is complex to make and is subject to numerous mechanical breakdowns that make it unreliable. In addition, with a variable-geometry pump, there is no way of installing control redundancy in order to mitigate a potential breakdown thereof.
The use of a positive displacement pump that is driven solely by an electric motor has also been envisaged (reference can be made to publication U.S. Pat. No. 4,815,278, for example). Such a solution enables the quantity of fuel taken by the pump to be matched accurately to the fuel requirements of the combustion chamber during all stages of flight of the airplane. Nevertheless, driving a pump by means of an electric motor requires a high power electric motor to be used (by way of example, a motor having power of about 50 kilowatts (kW) for a turbomachine gear pump), with all the drawbacks that that involves in terms of electrical architecture (presence of bulky inverters, etc.).